Scanned probe microscopy saw its commencement at the atomic level with the invention by Binnig et al. of the scanning tunnelling microscope in the mid-1980's. In the scanning tunnelling microscope (STM), a tiny tungsten probe is maneuvered to within a nanometer above the surface of a conducting specimen, sufficiently close so that there is an overlap between the electron clouds of the atom at the probe tip and of the nearest atom of the specimen. When a small voltage is applied to the tip, electrons "tunnel" across the gap generating a small tunnelling current. The strength of that current is very sensitive to the width of the gap. Piezoelectric controls are used to control the motion of the probe and move it back and forth across the specimen while maintaining a constant gap between its tip and the specimen surface. The variations in voltage applied to maintain the probe properly positioned over the surface are electronically translated into an image of the surface topography.
The invention of STM has led to the development of a family of new scanned-probe microscopes, one of which is the atomic force microscope (AFM) which negates the need for a conducting specimen. In its first implementation, the AFM relied upon the repulsive forces generated by the overlap of the electron cloud at the tip's surface with electron clouds of surface atoms within the specimen. The tip was mounted on a flexible beam which maintained the tip pressed against the specimen surface with constant force as it was moved across the surface.
While a number of methods have been used to measure the movement of the AFM beam/tip, more recently, this has been done by a laser beam. The reflected laser beam is detected and enables beam movements to be converted to imaging signals.
A more recent development is an AFM that is based on the detection of an attractive force between a surface and a probe by its effect on the dynamics of a vibrating probe/beam arrangement. Commonly in such arrangements, a tapered tungsten wire is driven by a piezoelectric transducer mounted at its base to vibrate at close to the wire's resonant frequency. As the tip of the wire is moved across a surface, vibration amplitude changes occur as a result of the attractive forces. The changes in the vibration amplitude are sensed by an interferometric laser arrangement.
A block diagram of an AFM using interferometric laser detection is shown in FIG. 1, with a probe arrangement 10 being emplaced over the surface of specimen 12. Changes in oscillation amplitude of probe arrangement 10 are sensed by a laser heterodyne interferometer 14 that provides an output signal to a feedback generator 16. As an output signal changes with respect to an applied reference signal, feedback generator 16 provides position control signals to a piezoelectric position control unit 18. Those control signals cause a piezoelectric unit within position control element 18 to move the specimen so that the vibration amplitude is stabilized and, hence the force gradient. The fluctuations in the feedback potential are converted into a profile of the surface being investigated.
Another variation of the AFM with which the invention can be used is the electrostatic-force microscope where the vibrating probe bears an electric charge and its vibration amplitude is affected b elecrostatic forces resulting from charges in the sample. One form of AFM is a magnetic-force microscope (MFM). In an MFM, a magnetized nickel or iron probe is substituted for the tungsten or silicon needles used with other AFM's. When the vibrating probe is brought near a magnetic sample, the tip feels a magnetic force that changes its resonance frequency and hence its vibration amplitude. The MFM traces magnetic-field patterns emanating from the specimen.
The magnetic force components sensed by an MFM probe result from the interaction of the total magnetic dipole moments at the tip of the probe and the specimen. They are further dependent upon the influence of tip-related magnetic fields on the local magnetic moments of the specimen. The lateral resolution of the probe depends critically on the interaction volume constituted by the sample and the tip. For planar magnetic media, this interaction volume is determined primarily by geometric and magnetic properties of the probe tip. Thus, in order to obtain lateral resolutions below 1000 Angstroms, current tip sizes in the range of 1000 Angstroms are too large.
Recent theoretical calculations by Wadas in the Journal of Magnetism and Magnetic Materials, Vol. 71, p. 147 (1988) and Vol. 72, p 292 (1988) show that even for probe tip sizes below 1000 Angstroms, detection sensitivity of the magnetic forces can be greatly improved by optimization of probe tip shape. Those calculations suggest that only the first 100 Angstroms of magnetic material at the tip are effective in contributing to magnetic force interactions that yield high resolution information. The remainder of the magnetic material at the probe tip contributes a background force due to long-range interactions between the magnetic cantilever and the specimen. These interactions are detrimental to sensitivity and spatial resolution and may induce domain wall motion in soft magnetic materials and instability in the tip-to-specimen separation.
Currently, magnetic sensor probe tips are fabricated by an electrochemical etching technique using a ferromagnetic wire material (such as nickel, iron, or cobalt). Essentially the method comprises etching the tip of the wire until it approximates a point. This method does not provide control over the geometric shape of the tip below a 1000 Angstrom radius. Such probe tips, further, have an unnecessarily large amount of magnetic material with a complicated domain structure. In essence, subtractive processes for creating MFM and other AFM probe tips, do not today, provide the desired atomic-level resolution capability.
The prior art is replete with many additive techniques for deposition of both magnetic and nonmagnetic materials. High energy electron and laser beams are also used to enhance local deposition of materials onto a surface from a gaseous environment. Those systems generally are employed to deposit a pattern onto a substrate, such pattern derived by selective deposition of one or more components of the gaseous environment through which the energy beam is being directed. In U.S. Pat. No. 4,382,186 to Denholm et al., a process is described which employs finely focussed electron beams to effect physical, chemical and other changes on the surface of a substrate. In U.S. Pat. No. 4,670,291 to Mori et al. The injection of exotic atoms into a semiconductor substrate is achieved through the use of an electron beam irradiation scheme.
Additive techniques have been used in the prior art to create ultra-fine ASM crystalline probe tips. In U.S. patent application, Ser. No. 07/568,286 to Bartha et al, entitled "Method of Producing Micromechanical Sensors For the AFM/STM Profilometry and Micromechanical AFM/STM Sensor Head", an additive method is described for the fabrication of a cantilever beam and crystalline tip. In U.S. patent application, Ser. No. 07/568,451 to Bayer et al, entitled "Method of Producing Ultra Fine Silicon Tips For the AFM/STM Profilometry", silicon is employed as the tip material. Once the silicon tip structure is formed by masking, the tip shaft is thinned through an etch procedure. Tip structures having horizontal protrusions are also disclosed. In U.S. patent application Ser. No. 07/568,306 to Bayer et al, and entitled "Method of Producing Micromechanical Sensors for the AFM/STM Profilometry and Micromechanical AFM/STM Sensor Head", there is disclosed a method for constructing low-mass microcantilever beams with integrated silicon-based tips. It is also disclosed that such tips may carry a metallic coating.
In U.S. patent application, Ser. No. 07/619,378 to Nyyssonen, entitled "Micro Probe-Based CD Measurement Tool", a silicon-based micro probe tip is shown which includes lateral protuberances to enable the probe to detect sidewalls of trenches in which it is emplaced. Each of the above-noted patent applications is assigned to the same assignee as in this application. A further reference, authored by Nyyssonen which speaks to multi-point probe tips can be found in a brief article entitled "Micro Probe-Based CD Measurement Tool", IBM TDB, Vol. 32, No. 7, December 1989, page 168.
Other references showing various-shaped tips comprised of crystalline material can be found in the following prior art publications: "Ion Milled Tips For Scanning/Tunnelling Microscopy", Biegelsen et al., Applied Physics Letters, Vol. 50, No. 11, 16 March, 1987, pp. 696-698; "Platium/Iridium Tips With Controlled Geometry For Scanning/Tunnelling Microscopy", Musselman et al., Journal of Vacuum Science Technolgy A, Vol. 8, No. 4, July/August 1990, pp. 3558-3562; "Scanning/Tunnelling Engineering", Schneiker et al., Journal of Microscopy, Vol. 152, Part II, November, 1988, pp. 585-596; and "Mono-Atomic Tips for Scanning/Tunnelling Microscopy", Fink, IBM Journal of Research and Development, Vol. 30, No. 5, September 1986, pp. 460-465.
In U.S. Pat. No. 4,605,566 to Matsui et al., a film is deposited on a semiconductor substrate by passing a gas containing an element over the substrate and then irradiating a determined portion thereof with an electron beam. The gas decomposes and the element is precipitated onto the substrate so as to form a desired pattern. Chromium, molybdenum, aluminum, and tungsten containing organometallics are disclosed. In Japanese Patent 87-295,868/42 the use of organometallic compounds is described for producing a film of group VIII metals on a substrate. Electron beams have also been used to enable surface analysis of substrates (e.g., see Kuptsis, IBM Technical Disclosure Bulletin, Vol. 13. No. 9. February, 1971. pp. 2497-2498). More recently, it has been found that iron, nickel, and palladium may be deposited on silicon directly from metallocenes. This is in contrast to other energy-beam assisted deposition systems wherein metal carbonyls and metal alkyls are employed as organometallic source compounds. Such actions are described by Stauf et al. in "Patterned Photo Assisted Organometallic Deposition of Iron, Nickel and Palladium on Silicon", Thin Solid Films, 156 (1988) pp. L31-L3. In specific, Stauf et al. indicate that depositions of metals from the decomposition of cobaltocene, nickelocene, and ferrocene were achieved by both conventional pyrolysis and by single-photon (non-thermal) decomposition or plasma-assisted deposition. Photon assisted deposition, as well as electron beam deposition, are essentially non-thermal in character.
Accordingly, it is an object of this invention to produce an AFM probe tip through the use of an additive process.
It is another object of this invention to produce a high aspect ratio probe tip of precise nanometer-scale dimensional characteristics.
It is yet another object of this invention to provide an electron beam and plating deposition method for producing probe tips.